1. Field of the Invention
The present invention relates to a method of producing a sintered body of a material for thermoelectric element, which is preferably used to prepare thermoelectric elements for a thermoelectric module that is a temperature control device using the Peltier effect.
2. Disclosure of the Prior Art
As shown in FIGS. 12A and 12B, a conventional thermoelectric module 100 is provided with an arrangement of N-type and P-type semiconductor elements 110, 120 as thermoelectric elements, which are arranged in a matrix manner such that each of the N-type semiconductor elements 110 is disposed adjacent to the P-type semiconductor element 120 through a required space, upper electrodes 130 disposed on a top surface of the arrangement to connect between adjacent semiconductor elements 110 and 120 according to a first circuit pattern, lower electrodes 140 disposed on a bottom surface of the arrangement to connect between adjacent semiconductor elements 110 and 120 according to a second circuit pattern different from the first circuit pattern, and ceramic plates 150 such as sintered alumina plates bonded to the upper and lower electrodes 130 and 140.
For example, as shown in FIG. 12B, when direct current is supplied to the thermoelectric module 100, each of the upper electrodes 130 has the flow of electricity from N-type semiconductor element 110 toward the P-type semiconductor element 120, and on the other hand each of the lower electrodes 140 has the flow of electricity from the P-type semiconductor element 120 toward the N-type semiconductor element 110. At this time, the upper electrodes 130 absorb heat from the surroundings through the ceramic plate 150, and the lower electrodes 140 radiate heat to the surroundings through the ceramic plate 150. Therefore, the thermoelectric module 100 works as a kind of heat pump for pumping heat from one side to the opposite side thereof, which is usually known as the Peltier effect. According to this principle, it is possible to use the thermoelectric module 100 as a temperature control device for electronic parts or circuit boards.
The thermoelectric elements 110, 120 can be produced according to the following method disclosed in Japanese Patent Early Publication [KOKAI] No. 9-321357. That is, as shown in FIG. 13, an ingot of a material for thermoelectric element is ball-milled in a non-oxidation atmosphere to obtain a powder thereof After charging the powder into a capsule made of a metal material such as aluminum, the degassing of the capsule is performed to obtain a billet for extrusion. As shown in FIG. 14, an extrusion step is then performed by use of an extrusion die 70 to reduce a diameter of the billet 72. In FIG. 14, the numeral 76 designates the powder of the thermoelectric-element material charged in the capsule 74. Next, a heat treatment is performed to sinter the powder in the worked billet. By removing a resultant sintered body from the capsule, a thin rod of the sintered body of the thermoelectric-element material is obtained.
In the above method, since the ingot is previously ball-milled, it is possible to reduce the segregation of alloying elements in the ingot, i.e., nonuniform distribution of alloying elements in the ingot. As a result, variations in thermoelectric performance and mechanical properties of the thermoelectric elements decrease. In addition, as compared with a case that the thermoelectric elements are directly cut from the ingot, it is possible to remarkably reduce the occurrence of cracks or chipping of the thermoelectric elements. Moreover, since the mechanical strength of the thermoelectric elements is improved by the heat treatment, the yields of the thermoelectric-element material increase.
By the way, the heat-pump performance of the thermoelectric module 100 highly depends on the thermoelectric performance of the thermoelectric elements 110, 120. The thermoelectric performance can be improved by providing the uniform distribution of alloying elements in the ingot, reducing amounts of impurities trapped in the thermoelectric element, and/or increasing a degree of orientation of a specified crystal plane of the thermoelectric-element material. In the above-described method, since the ingot is ball-milled, the uniform distribution of alloying elements can be achieved. However, the amounts of impurities in the obtained powder of the thermoelectric-element material generally increase. Therefore, there is a limitation in improvement of the thermoelectric performance.
On the other hand, when the degree of orientation of the specified crystal plane that is the so-called "C" crystal plane of the thermoelectric-element material is increased, the thermoelectric performance can be remarkably improved. That is, when direct current is supplied to the thermoelectric element along the crystal orientation, the improved thermoelectric performance is obtained. In the above method, since the ingot is ball-milled, the powder of the thermoelectric-element material is in random orientations of the "C" crystal plane. Although the degree of orientation of the "C" crystal plane can be improved to some extent by performing the extrusion step to the capsule having the powder therein, it is not sufficient to obtain excellent thermoelectric performance.